Starcraft II doesn't really make use of quads. It wants really good single core performance, plus at least one more core available.
When you look at CPU charts for Starcraft II, they pretty much threaded efficiency. Unfortunately, the best single threaded CPUs are also more expensive.
http://www.techspot.com/review/305-starcraft2-performance/page13.html
Note that a basic i3 @ 3.06 performs about on par with an AMD Phenom x4 @ 3.4, and scales well with CPU speed. An i3 Overclocked to ~4.0 GHz is the best CPU choice if you're on a tight budget.
The extra cache and memory controller configuration of the i5 750 at it's stock 2.66 GHz performs puts it in quite a good position, and when overclocked, you can see why this CPU was a popular option among gamers.
Even with that the case, I'd caution against buying the i5 right now. The sandy bridge CPUs are out, but the mobo issues are holding them back. the sandy bridge i3s aren't really an optioon, as you can't OC them and they have no turbo mode. But the i5 2400 is fast out of the box, with 3.1 GHz / 3.4 Turbo and faster per clock than the i5 750 / 760. Even stock a 2400 is about the speed of an overclocked i5 750, but consumes less power and it's about the same total price with motherboard and CPU prices near identical to the i5-760.
http://www.anandtech.com/show/3871/the-sandy-bridge-preview-three-wins-in-a-row/11
has Starcraft II and WoW benchmarks for these CPUs (the i7 8xx is essentially the same CPU as the 7xx, consider it slightly overclocked. It enables hyperthreading as well, but that feature is worth nothing to these particular games.)
I have an i3 540 overclocked to slightly over 4.0GHz. Not a quad, but more games prefer a fast dual to a slower quad, though there are a few exceptions.